In servers, routers, or storage devices that handle high-speed digital signals of several Gbit/s or more, a differential signal transmission system is employed for transmitting signals among such equipment or between circuit boards within such equipment. The differential signal transmission system transmits a signal in a two-phase style of signal on two conductors: one normal phase and the other 180-degree inverted phase. On the receiving end, the differential between the received two-phase signals are combined to be outputted. The directions of signal currents flowing along the two conductors are opposite each other; therefore, the electromagnetic wave that the transmission line may emit is small. Further, noise induced from the outside is superposed evenly on each of the two conductors; therefore, the effect of noise can be eliminated by combining the differential between them at the receiving end. For these reasons, high-speed digital signal transmission systems often use the differential signal.
As a differential signal cable to be used for transmitting differential signal, a twisted pair cable may be cited. The twisted pair cable is comprised of two insulated wires, which are conductors covered with insulator and twisted into a pair. The twisted pair cable is an economical cable with excellent circuit balance properties and is easy to bend; the cable therefore is used widely by preference for signal transmission over a medium distance. The twisted pair cable however has large signal attenuation. Because of this, a system that uses a twisted pair cable needs large electric power (about 6 to 10 times of that in a twinax cable as will be described later) for signal processing to compensate the signal attenuation. A twisted pair cable of a general style has no metallic layer thereon that will function as a shield. Therefore, the cable tends to be affected by the existence of a nearby-metallic body inviting a problem of unstable characteristic impedance of the cable. Further, the difference of the physical length between insulated conductors in a pair is large because the construction of the twisted pair cable is a strand of two insulated wires. Therefore, the effect of skew becomes large when the twisted pair cable is covered with a metallic conductor layer that works as a shield. Because of these, signals in the high-frequency region of several GHz tend to deform in their waveforms. Therefore, the twisted pair cable has been seldom used as a transmission line that is to convey signals with a rate of several Gbit/s.
On the other hand, there is a cable in which two insulated wires are arranged parallel without twisting and are covered with a shield (hereinafter referred to as a twinax cable). The twinax cable has smaller signal attenuation in the high-frequency range compared to twisted pair cables. Since a shield is provided over two insulated wires, the twinax cable is stable in its characteristic impedance even if a metallic body is placed close to the cable and the susceptibility to noise is low. Because of these advantages, the twinax cable has been used for transmission of relatively higher-speed signal over short distances. As the construction of the shield, tapes with conductive layer and coverings of braided wires are applied. Instead of covering with a shield, a drain wire may be incorporated.
For example, FIG. 5 is a sectional view of the twinax cable 50 disclosed in JP 2002-289047 A as an example of twinax cables, in which two insulated wires 533, each of which is a signal conductor 531 jacketed with an insulator 532, are lapped or longitudinally lapped with a shield tape 535, which is a laminate of a polyethylene tape 538 to which a metallic foil 537 such as aluminum is bonded. Between the shield tape 535 and the insulated wire 533, a drain wire 534 is arranged so that the drain wire will contact the metallic foil 537 of the shield tape 535, and is grounded.
FIG. 6 is a sectional view of an example of the twinax cable 60 disclosed in JP 2003-346566 A. Two insulated wires 633, each of which is a signal conductor 631 jacketed with an insulator 632, have fusion layers 636 severally thereon. Two insulated wires 633 are bonded mutually via the fusion layer 636 and covered with a shield tape 635, which is a laminate of a metallic foil 637 such as aluminum, applied over the bonded configuration. In the twinax cable 50 illustrated in FIG. 5, a positional deviation appears between the insulated wires 533 because of slipping caused by a repeated bending or similar handling of the cable. In the twinax cable 60 illustrated in FIG. 6, no slipping occurs in contrast since the insulated wires 633 are mutually fused. It is indicated in the literature that the skew, which is the difference of signal propagation time between two signal conductors, will be reduced by such configuration. Increased skew deforms the digital signal waveform obtained by combining the signal differentials at the receiving end. As a result of this, the signal quality degrades even if the skew is several pico seconds when a transmission system handles high-speed signals of 10 Gbit/s or equivalent rate.
FIG. 7 is a sectional view of an example of the twinax cable 70 disclosed in JP 2001-035270 A. Signal conductors 731 are overall-jacketed with an insulator 732 and lapped or longitudinally lapped with a shield tape 735, which is comprised of a metallic foil tape.
FIG. 8 is a sectional view of an example of the twinax cable 80 disclosed in JP 2007-026909 A. Insulated wires 833 are covered with a foamed tape 839 and covered with a shield tape with a drain wire 834 longitudinally placed on the foamed tape 839.